It is known that various organic solvents have the ability to swell thermoplastic polymers such as polyvinyl chloride (hereinafter referred to as "PVC"). This property has been used to incorporate or infuse into the surface region of the article ingredients which will impart protection against the degradative effects of the atmosphere, especially ultraviolet light and oxygen.
It is common practice to protect articles such as PVC siding against the degradative effects of ultraviolet light by employing high levels of opacifying materials such as titanium dioxide as ultraviolet light screens. It is also known to incorporate ultraviolet light stabilizers in such articles to enhance their outdoor weatherability.
U.S. Pat. No. 3,297,462 disclosed a process for rendering polymeric films resistant to the degradative effects of sunlight by immersing them in a solution of a benzophenone derivative in a mixture of diethylene glycol and glycerine followed by washing first in diethylene glycol and then in water. The polymers stated as being suitable for such treatment are those having a zero strength temperature of at least 200.degree. C. This requirement excludes a number of useful polymers such as PVC. PVC films subjected to this treatment produced hazy samples due to softening and exhibited little or no improvement in weatherability.
Belgium patent No. 612,206 discloses a process for treating rigid PVC objects in order to stabilize them against the action of sunlight by immersing them in a swelling solvent containing a light stabilizing agent followed by evaporation of the solvent. This procedure is undesirable in that stabilizer is left on the surface of the objects, environmental hazards are created, and a product having a poor surface appearance is obtained.
U.S. Pat. No. 3,892,889 relates to a method of applying ultraviolet light stabilizers to polycarbonates in the form of an organic solvent solution followed by drying. Here too, the resultant polymers are marred by the presence of stabilizers deposited on the surface.
U.S. Pat. No. 4,323,597 pertains to a process for producing polycarbonates stabilized against ultraviolet light effects. The surface of the polycarbonate is contacted with a liquid carrier which is non-aggressive towards and wets the surface. There is no indication that the process is applicable towards other polymers such as PVC.
U.S. Pat. No. 3,519,462 describes the treatment of various polymers such as PVC with a solution of ultraviolet light stabilizers in a diol or triol. These solvents are quite inefficient, and further there is no mention of the need for removal of the excess solution.
Despite considerable past research aimed at protecting thermoplastic polymers such as PVC from the degradative effects of sunlight, there still exists a need for a method of treatment of polymer articles to provide such protection. Desirably, such process should be able to be carried out as a batch or continuous process (the latter is preferred) and be environmentally safe (i.e. the process should be capable of being carried out at ambient temperature and the solvent concentration in the atmosphere should be below that permitted by regulation). In a typical PVC extrusion assembly line process, the maximum practical upper limit for contact time of PVC articles with stabilizing agents is about two minutes. Accordingly, the process must be capable of causing the requisite minimum of stabilizer to be incorporated in the surface region of the article (such minimum appears to be about 0.1 per 100 g of article.)
It is an object of this invention to provide a method for incorporating various types of ingredients such as ultraviolet light stabilizers, antioxidants, anti-static agents, pigments, etc. into the surface region of polymer articles, especially those fabricated from thermoplastic polymers such as PVC.
It is also an object to render articles of solid PVC resistant to degradation by actinic radiation.
It is an additional object to achieve this end without leaving the surface of the articles softened, streaked, or coated with any of the ingredients incorporated therein.
It is a further object to provide an environmentally safe method which can be carried out in a continuous mode and which is adapted to commercial time restraints.